JP2722769B2 - Gain control circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gain control circuit used for a magnetic recording / reproducing device, a television device, and the like.

2. Description of the Related Art Conventionally, in a gain control circuit, when a gain is fixed without depending on a signal level, it is widely performed that an arbitrary control voltage is externally applied to a control signal input terminal to fix the gain. .

 FIG. 3 shows a block configuration of a conventional gain control circuit.

 The principle of operation of FIG. 3 will be described in detail below.

According to this method, a control reference voltage V _REF and a control voltage V _CTL are input from a fixed power supply to a gain control circuit 3, and an output thereof, that is, a signal obtained by detecting a difference between both inputs as a gain is input to a detection circuit 5. Then, a closed loop is formed in which the output voltage of the detection circuit 5 is further fed back to the input control voltage through the buffer amplifier 6. In a normal case, the feedback is performed by forcibly applying the detection voltage from the outside. The gain is fixed so that it does not work.

In such a conventional system, the gain control circuit 3 is externally provided.
The control voltage V _CTL applied to the control voltage does not have sufficient relative accuracy with respect to the control reference voltage V _REF, and the control voltage V _CTL is forcibly applied to the control voltage V _CTL to fix the gain (fixed gain). The problem is that the gain fluctuates depending on the fluctuation of the ambient temperature or the power supply voltage.

The present invention has been made in view of the above, increasing the relative accuracy of the control voltage V _CTL with respect to the reference voltage V _REF, and its object is to provide a low gain control circuits setting errors of fixed gain.

Means for Solving the Problems In order to solve the above problems, the gain control circuit of the present invention,
A gain control circuit (3) for controlling a gain by comparing an input reference voltage (V _REF ) with a control voltage (V _CTL ); and a detection circuit (5) for detecting an output level of the gain control circuit. ,
The other end of the connected to the output of the detection circuit to obtain a smooth to the control voltage output signal of the detector circuit capacitor (7), the first resistor has one end connected to power supply terminal (10) (R _L) A first transistor (Q ₈ ) having a collector connected to the reference voltage, a second transistor (Q ₇ ) having the control voltage input to the base, and the first and second transistors A first differential amplifier comprising a first current source for supplying a current to the common emitter connection point of the first and second resistors (R _X ) having one end connected to the power supply terminal, and A constant voltage generating means (4) for supplying a current of a second current source that outputs a current equivalent to the current of the current source to generate a predetermined DC voltage between terminals of the second resistor; 2
A third transistor (Q ₉ ) having a base connected to the other end of the first resistor, and a fourth transistor (Q ₉ ) having a base connected to the other end of the first resistor.
Transistor (Q ₁₀ ), gain control terminal (9)
And a third current source (Q ₁₃ ) that operates by an input provided to supply a bias current to a common emitter connection point of the third and fourth transistors, and controls a terminal voltage of the capacitor by an output signal. And a second differential amplifier.

According to the present invention, the first current source and the second current source operating at the same current allow the voltage between the terminals of the first resistor _{RL and} the terminal of the second resistor RX to be _adjusted . The correlation is obtained, and the output voltage of the first differential amplifier (Q ₇ , Q ₈ ) and the output voltage of the constant voltage generating means 4 can have a relative relationship. And
The terminal voltage of the capacitor 7 is reduced by a negative feedback loop including a first differential amplifier for comparing the control reference voltage V _REF and the control voltage V _CTL and a second differential amplifier (Q ₉ , Q ₁₀ ). By controlling, the relative value between the DC potential of the control voltage _VCTL and the reference voltage _VREF can be set with high accuracy, and the gain of the gain control circuit can be fixed with high accuracy.

Embodiment FIG. 1 shows a circuit diagram of an embodiment of the gain control circuit of the present invention. In FIG. 1, a control reference voltage V _REF and a control voltage V _CTL
The output terminal of the gain control circuit 3 for feedback input of the
Connect to the input terminal of The output terminal of the detection circuit 5 is connected to the detection capacitor 7 and the buffer amplifier 6. Output sources of the control voltage V _CTL and the control reference voltage V _REF are respectively connected to a non-inverting input terminal and an inverting input terminal of the differential amplifier 1, and this output terminal is connected to an inverting input terminal of the differential amplifier 2. And an output terminal thereof is connected to an output terminal of the detection circuit 5. Note that a DC voltage source 4 is connected to a non-inverting input terminal of the differential amplifier 2.

FIG. 2 shows the configuration of FIG. 1 more specifically. That is, connect each collector directly to the power supply or
The differential amplifier 1 is formed by connecting the respective emitters of the NPN transistors Q ₇ and Q ₈ connected via R _L to the constant current source I ₀ in common. Next, they have a common base, and each emitter has a resistor R ₂ , R ₃
Through connecting the collector of the transistor Q ₁₃ of PNP transistor Q _13, Q ₁₄ connected to the power source to the emitters of the PNP transistors Q _9, Q _10, and the respective collectors of the transistors Q _9, Q _10, each NP with emitter grounded and base connected together
N A resistor connected to each collector of transistors Q ₁₁ and Q ₁₂ and the base of transistor Q ₉ connected between the power supply terminal and ground.
Connect between R _X and DC current source I ₀ . On the other hand, transistor Q
₁₀ bases are transistors constituting the differential amplifier 1
Connect to Q ₈ collector. Further, through the collector and base resistors R ₄ transistors Q _14, and an emitter grounded, and is connected to the collector of NPN transistor Q ₁₅ which is connected a resistor R ₅ to the base. Furthermore, by connecting the collector and base of the transistor Q _11, and the differential amplifier 2. Next, an NPN with each collector connected directly to the power supply or via a resistor _RL
It has transistors Q ₄ and Q ₃ and transistors Q ₅ and Q ₆ , respectively, and the emitters of the transistors Q ₃ and Q ₄ and the transistor Q
_5, each emitter of Q _6, respectively connected to the collector of NPN transistor Q _1, Q ₂ connected to the emitters to the constant current sources I _0. The transistors Q _1, resistors R ₁ between each base Q ₂ 'connected two series, the DC power source V ₁ and the ground connection to the midpoint, also connects the resistor R _E between the emitters. The transistor
The collectors of Q ₃ and Q ₆ are connected to the inverting and non-inverting input terminals of the differential amplifier 8, respectively, and the bases of the transistors Q ₄ and Q ₅ are connected to form a gain control circuit. The base of the base of the transistor Q ₈ of the transistor Q _3, Q _6, also bases of the transistors Q ₇ of the transistor Q _4, Q ₅ is connected.

The operation of the embodiment shown in FIG. 2 will be described below. When fixing the gain, setting the gain fixed OFF control terminal 9 than the voltage which can turn on the transistor Q _15. Whereby a current flows through the transistor Q ₁₄ and the Q _13, a differential amplifier 2 operates.

On the other hand, the base voltage V _X of the transistor Q ₉ is a current amplification factor h _FE of the transistor itself and is sufficiently large (a value that is regarded as mathematically infinite), when the power supply voltage is Vcc, the following ( It is expressed by the equation 1).

_{V X = Vcc-R X I} 0 ...... (1) the differential amplifier output of 2 detection circuit 5, a buffer amplifier 6, since it is negatively fed back through the differential amplifier 1, is also the base of the transistor Q ₁₀ (1) The voltage is equivalent to the equation. Thus, current flows I _X represented by the following formula (2) to the transistor Q _8.

I _X = (Vcc−V _X ) / R _L (2) The following equation (3) is derived from both equations (1) and (2).

I _X = R _X I ₀ / R _L (3) The base of the transistor Q ₈ is the same as the transistor Q ₃
With the bases of Q ₆ are connected to the control reference voltage V _REF, and, since the base of the transistor Q ₇ is connected to the base and the output terminal of the buffer amplifier 6 of the two transistors Q ₅ and a transistor Q _4, the transistor Q ₃ and the current represented by each (3) in the Q ₆ is flowing.

Condition the gain of the gain control circuit 3 becomes the maximum, the current flowing through the transistor Q ₃ and the transistor Q ₆ is when I _0. If the gain at this time is G _MAX [dB], the fixed gain G _CONST [dB] is It is expressed as Therefore, the following equation (5) is obtained from both equations (3) and (4).

Output detection circuit 5 of the differential amplifier 2, a buffer amplifier 6, or more, which is fed back to its inverting input through a differential amplifier 1, the difference component of the control reference voltage V _REF and the control voltage V _CTL base voltage of the transistor Q ₉ I'm trying to follow V _X ,
The relations of the expressions (1) to (5) always hold. Therefore, the resistors R _E and R _L are used so that the gain G _MAX is not _affected by temperature and power supply fluctuations.
If the configuration of the differential amplifier 8 is determined, the gain becomes (5)
As can be seen from the equation, it is determined simply by the ratio of the resistances R _X and R _L.

From the above, if the resistors R _X and R _L are composed of the same type of resistors, the difference between V _REF and V _CTL is controlled by the feedback loop so that the gain becomes the value represented by the equation (5), and the temperature change And little fluctuation due to power supply fluctuation.

As described in detail above, according to the present invention, the gain of the gain control circuit can be fixed, and the gain can be made hardly affected by changes in temperature or power supply voltage, which is extremely useful. It is.

[Brief description of the drawings]

1 is a circuit diagram of a gain control circuit according to an embodiment of the present invention, FIG. 2 is a circuit diagram of one embodiment specifically realized using bipolar transistors, and FIG. 3 is a block diagram of a conventional gain control circuit. is there. 1,2,8... Differential amplifier, 3... Gain control circuit, 4... Constant voltage generating means having temperature characteristics and power supply fluctuation characteristics equivalent to the output DC component of differential amplifier 1, 5. , 6: buffer amplifier, V _REF: control reference voltage, V _CTL: control voltage, 7: detection capacitor, 9: gain fixed on / off control terminal, 10: power supply terminal, 11: ground terminal, V ₁ … arbitrary constant voltage source, 12… input capacitor, 13… output terminal, 14… fixed gain on / off switch, 15… arbitrary constant voltage source, Q _{1 to} Q ₁₅ … transistor I ₀ ... constant current source, R _{1 to} R ₅ , R _E , R _L , R _X ... resistance.

Claims (1)

(57) [Claims] 1. A gain control circuit for controlling a gain by comparing an input reference voltage and a control voltage, a detection circuit for detecting an output level of the gain control circuit, and an output of the detection circuit. A capacitor for smoothing an output signal of the detection circuit to obtain the control voltage, and a collector connected to the other end of a first resistor having one end connected to a power supply terminal and the reference voltage being input to a base. A first differential amplifier comprising: a first transistor, a second transistor to which the control voltage is input into a base, and a first current source for supplying a current to a common emitter connection point of the first and second transistors. Supplying a current of a second current source that outputs a current equivalent to the current of the first current source to the other end of the second resistor having one end connected to the power supply terminal; A predetermined DC voltage between the terminals A constant voltage generating means for raw, third transistor whose base is connected to the other end of said second resistor, a fourth whose base is connected to the first end of the resistor
And a third current source which is operated by an input given to a control terminal for fixing a gain and supplies a bias current to a common emitter connection point of the third and fourth transistors. And a second differential amplifier for controlling a terminal voltage.